add doxygen configuration

Tested on SLC6 node (provides doxygen v1.6.1 and dot)

Testing the doxygen configuration.
If we think it's useful, I will go ahead and change all the relevant
comments to the doxygen syntax (now only a few of them are
actually shown in the html output).
See output [at this link](https://test-gerbaudo.web.cern.ch/test-gerbaudo/doc_SusyNtuple/html/)
In particular, the inheritance tree might be useful, see [here](https://test-gerbaudo.web.cern.ch/test-gerbaudo/doc_SusyNtuple/html/inherits.html)

The command used to generate this doc is `doxygen doc/doxygen.conf`